Abstract: Purpose: To investigate the effect of soybean oligopeptides on blood pressure and plasma angiotensin levels in rats. Methods: Soybean oligopeptides were prepared by enzymatic treatment, and its angiotensin I converting enzyme (ACE) inhibitory activity was evaluated both in vitro and in vivo experiments. Rats were divided into six groups: normal control (Wistar-Kyoto (WKY) rats with normal blood pressure rats fed high-dose soybean oligopeptides), negative control (spontaneously hypertensive (SHR) rats), positive control (SHR rats fed captopril), low, medium and high-dose groups (SHR rats fed 0.90, 1.80, 4.50 g/kg soybean oligopeptides). The feeding experiment lasted for 30 days. The changes in blood pressure, heart rate, urinary protein content and plasma angiotensin II concentration were determined during the experimental period. Results: The optimal conditions for enzymatic hydrolysis of soybean protein were found to be hydrolysis at 50 ℃ for 4 h with a mixture of alkaline protease and neutral protease at a dosage of 0.1% each. The percentage inhibition of ACE activity by soybean oligopeptides at 10 mg/mL prepared using the optimized conditions was up to 71.2%. After 30 days of feeding, there was no significant difference in blood pressure of WKY rats (P > 0.05), while the blood pressure of SHR rats had a decreasing trend in all dose groups of soybean oligopeptides. High-dose soybean oligopeptides significantly reduced blood pressure and mass concentration of angiotensin II in SHR rats at the end of the fourth week (P < 0.05), but had no significant effect on rat heart rate or urinary protein content (P > 0.05). Conclusion: Compared with soybean protein, soybean oligopeptides had better inhibitory effect on ACE activity, and could reduce blood pressure and plasma angiotensin II levels in SHR rats but without significant effect on blood pressure in normal rats. Therefore, soybean oligopeptides can exert antihypertensive effect perhaps by inhibiting ACE activity.

Key words: soy protein, soy oligopeptides, hypotensive effect, angiotensin I converting enzyme, angiotensin